The present invention relates to an insufficient pressure detecting device for motor vehicle tires.
It is known that the tires for motor vehicle wheels require pre-established inflation pressures which vary according to the type and dimensions of the tire, the weight acting on the tire and the prevailing use of the motor vehicle. The said inflation pressure must be within well defined limits; in particular, when the said pressure drops below a lower limit, a deformation of the tire takes place with a consequent development of heat by friction. Thus, an increase of the temperature of the tire is produced, which increase can reach such high values as to produce irreversible alterations of the rubber mix and a progressive damage of the tire itself.
Moreover, since also the temperature of the air inside the tire increases, a corresponding increase of the inflation pressure takes place; therefore, the limit of insufficient pressure should be referred to a precise value of the temperature, and thus an insufficient pressure detector device should have a point of intervention as a function of the temperature itself.
Known detector devices are mounted inside the metal rim of the wheel and are substantially formed by a closed tight cavity filled with air, or another gas, at a pressure near the one which has to be signalled; a wall of the said cavity is formed by a resilient diaphragm one outer face of which is in contact with the air under pressure contained within the tire. Thus, this diaphragm is subjected, on one hand, to the pressure within the tire and, on the other hand, to the pressure within the tight cavity; this cavity is separated from the air contained within the tire by means of walls made of a material which is a good heat conductor, so that the gas contained within the cavity is substantially at the same temperature as the tire.
Connected to the diaphragm, which preferably is made of metal, is a system of electric contacts which are actuated by the diaphragm. Depending on the inflation pressure of the tire being respectively higher or lower than a pre-established lower limit value previously calibrated by means of more or less gas being introduced into the said tight cavity, the said electric contacts assume a first or a second reciprocal position and enable, in a corresponding electric circuit, signaling means which signal any eventual decrease of the pressure below the pre-established limit value.
The compensation of the inflation pressure increase produced only by the heating takes place inasmuch as also the gas contained in the said cavity is heated, which gas also undergoes a pressure increase so that on the opposed surfaces of the said resilient diaphragm two pressure variations are generated which are equal to one another and opposite in sign.
The devices of the type described, although operating in a satisfactory way, have some disadvantages.
Firstly, they are suitable for being mounted in a simple way only on wheel rims with tubless tires, i.e. with the sensible diaphragm facing directly into the space in which it is desired to detect the pressure, whilst in the case of tires with an air tube the said devices require the tube to be of a special configuration or to be provided with particular means for causing the air contained in it to lap the said sensible diaphragm. Secondly, the said devices require the said tight cavity to be filled during the manufacture with air, or another gas, having a pressure higher than the atmospheric pressure. This involves both a constructional complication and a certain risk that in the long time, owing to even the smallest manufacturing imperfections, the cavity might loose the gas through microslots, porosity or by diffusion through the walls, thereby giving rise to a lowering of the minimum limit of calibration and a consequent delay of the indication of insufficient inflation pressure.